A major challenge to advancing our understanding of how proteins fold is the development of an analytical theoretical model capable of calculating the quantities directly measured in both equilibrium and kinetic experiments. We have approached this problem experimentally by studying a small ultrafast folding protein, the 35-residue subdomain from the villin headpiece It is the smallest naturally occurring protein that autonomously folds into a globular structure, so it should have one of the simplest protein-folding mechanisms, which may therefore be amenable to understanding in depth by a simple theoretical model.[unreadable] An extensive set of structural, equilibrium, and kinetics data on the villin subdomain, including X-ray structures, heat capacity, tryptophan fluorescence quantum yield (QY), and natural circular dichroism spectrum (CD) as a function of temperature in both denaturants and viscogens, while the kinetic data consist of time courses of the QY from nanosecond laser temperature jump experiments as a function of temperature, denaturant concentration, and viscosity. The relative effect of 10 site-directed mutants on folding thermodynamics and kinetics (&#61542; values) was also studied. These data were analyzed with three different kinds of models see annual report on Theoretical Studies of Protein Folding and Function.